I thought you asked for non-boss monsters. Still, even with these "not high" ACs, it's better to aim at the lower save.
I do. 4 attacks dealing 1d8 each is no different than a cantrip dealing 4d8. I could consider critical hit chance but it's more math than I want to do and I think I don't need, because in these cases the spellcasters can use their attack cantrips to aim at AC too.
It's simetrical, spellcaster almost certainly have an item that increases spell save DC. Targets can also have magic armor os shields that increase AC as much as the magic weapon bonus. Moreover, I think it is easier to increase AC with magic items than to increase weapon attacks or saves.
It's simetrical. Spell targets could also have disadvantage in saves.
Spellcasters will never run out of cantrips.
Amazingly I did, my bad. There are 3 other examples there, and mainly, all NPCs built using the same PCs rules will have the problem I mentioned. Just take all PCs you ever created and look at their sheets. Unless you did an amazing job increasing all saves, there's a high chance AC is higher than the lower save. These PCs could be NPCs, bosses, villains. Every time someone asks me to point monsters as examples I think they forget it's possible to play non-dungeoncrawl adventures set in cities and dealing with non-monster enemies.
It was considered. Advantage tends to add +5 only if chances are 50% or near. The Helmed Horror is an example, without advantage the chances of avoiding a spellcaster DC 19 spell with +0 Str would be 10% without advantage vrs. spell, it is 19% because of the advantage.